The present invention relates to methods and apparatus for densifying cryogenic liquids and, more particularly, to a simplified system for densifying liquid for long term storage, or for use in propulsion systems as densified propellant or reactant.
Propulsion systems utilizing cryogenic liquid oxygen and/or hydrogen, such as the Space Shuffle, Atlas/Centaur, Delta, etc., are currently filled from the facility storage tanks and subsequently allowed to cool in the flight tanks in order to reject the heat absorbed by the liquid as a result of environmental heat leak, transfer line, and tank wall chill-down. The cooling of the liquid bulk is desirable in order to increase the liquid density so that more impulse mass can be stored in the tank, and also to reduce the liquid vapor pressure so that the tank operating pressure and tank weight is minimized.
The next generation of advanced reusable launch vehicle (RLV) propulsion systems requires significant improvements in vehicle performance and operational cost reduction in order to make travel into space economically feasible. Recent efforts toward achieving these goals have focused primarily on high performance rocket engines, lightweight composite structures, lightweight/durable thermal protection systems, and lightweight storage tanks. Except for the use of slush hydrogen (mixture of liquid and solid), improvements in cryogenic liquid propellant properties have not been actively pursued.
An alternative to slush propellant has been identified that is simple, low cost, and provides significant vehicle weight and operational cost reductions. The concept involves the super cooling, or densification, of liquid oxygen and hydrogen below the present operating temperature experience. Densification of cryogenic propellants improves fluid properties (density and vapor pressure), which subsequently results in smaller tanks (xcx9c11% LO2, andxcx9c7% LH2), and lower tank operating pressures. FIG. 1 illustrates the improvement in thermo-physical properties of liquid oxygen (LO2) as a function of sub-cooled temperature. As is clear from the chart, lowering the temperature of oxygen results in higher density and lower vapor pressure. The triple point of oxygen, at approximately 98xc2x0 R, is illustrated at the left, while the boiling point, at approximately 162xc2x0 R, is shown at the right.
These propellant attributes result in significant weight savings for new launch vehicles or increased payload capability for existing launchers. Vehicle sizing studies for the single-stage-to-orbit RLV indicate a total weight reduction between 15 to 30 percent due to propellant densification. FIG. 2 illustrates the savings realized by utilizing densified liquid oxygen and liquid hydrogen in the fuel tanks of a reusable launch vehicle, such as the Space Shuttle. More specifically, the use of densified fluids results in approximately a 17% reduction in vehicle gross lift-off weight (GLOW). In addition to large vehicle weight reduction propellant densification increases the storage time of the cryogenic liquids without venting between 4 to 10 fold. Also, the cost per pound of weight saved with densification is an order of magnitude less than with other weight savings candidates (aluminum-lithium tank, filament wound LH2 tank, composite structure, or advanced main engines).
Because propellant densification promises large vehicle weight reduction with low ground cooling unit investments cost, densification has been recognized as an enabling technology for future launch vehicle designs, or a significant performance improvement to existing reusable or expendable launchers. Currently, systems used to service the fuel tank of the space shuttle require an expensive 3-stage compressor to reduce a liquid nitrogen heat exchange bath temperature sufficiently to maintain liquid oxygen being densified at about 120xc2x0 R. With larger and more expensive compressors, the liquid oxygen temperature could be reduced even farther, although the slush point of liquid nitrogen at about 115xc2x0 R provides a lower limit.
The prior art method for generating sub-cooled cryogenic liquid fuel is based either on directly lowering the heat exchanger bath pressure, or lowering the bath temperature through the use of a refrigeration system. Both of these concepts require the use of rotating machinery and a significant external power source. The primary disadvantages of using rotating machinery (vacuum pump, compressor, expander, turbine, etc.) to generate low temperature cryogens are that such densification systems tend to be complex, they are less reliable, they require periodic maintenance/ground check-out operation, and they are relatively expensive.
A system for densifying liquid propellant is disclosed in U.S. Pat. No. 5,644,920, issued to Lak et al. The ""920 patent includes a heat exchanger wherein liquid propellant is cooled and thus densified. The heat exchanger bath is either the liquid propellant itself, or a different liquid. For example, propellant liquid oxygen may be cooled with even colder liquid oxygen, or with liquid nitrogen. The heat exchanger bath fluid is cooled using a vacuum pump or compressor that lowers the bath pressure such that it boils at a lower temperature. The use of a vacuum pump or compressor to cool the heat exchanger bath, however, introduces significant complexity and cost to the densification system. For example, it is estimated that a multi stage compressor necessary to cool a liquid nitrogen heat exchanger bath to 120xc2x0 R, and having a sufficient flow capacity for cooling liquid propellant stored in the fuel tank of rocket, costs on the order of several million dollars. In addition, the relatively large and complex compressors and associated motors require constant maintenance, and checkout. Furthermore, a relatively large power supply is required to support the compressor. And finally, the introduction of high-voltage machinery with rotating parts in the presence of various propellants at a launch site introduces an inherent safety risk. Thus, while the system disclosed in the ""920 patent functions very effectively for its intended purpose, a simpler, safer, and more efficient approach would be desirable for cost-critical applications.
Although densified/subcooled liquids are highly desirable for propulsion systems to reduce launch vehicle size and operating cost, densified liquids also benefit ground and space based storage systems by reducing the size of the storage tank or by increasing the storage time.
The present invention provides a liquid densification system that is simple, inexpensive and safe. In contrast with prior densification systems, no expensive compressor is required to reduce the temperature of the heat exchange bath, and liquid being cooled, within the heat exchanger. Instead, the heat exchanger utilizes a primary, inert component and a secondary component for the heat exchange bath. The primary component fills a majority of space around the heat exchange tubes, while the secondary component is colder and is injected when needed. This arrangement permits the temperature of the liquid to be reduced in a very short time without the need for a high-voltage power supply and maintenance for the heat exchanger in and around the launch vehicle.
In one aspect of the present invention, a system for cooling and densifying a liquid includes an inlet supply line and a heat exchange tank having a plurality of heat exchange tubes therein, each tube being in fluid communication with the in let supply line. An outlet line is in fluid communication with each of the heat exchange tubes. A first inlet conduit connects to the heat exchange tank to introduce a first component of a heat exchange bath to the interior of the heat exchange tank, and into contact with the exterior of heat exchange tubes. A second inlet conduit connects to the heat exchange tank to introduce a second component of the heat exchange bath to the interior of the heat exchange tank, and into contact with the exterior of heat exchange tubes. The densified liquid may be directed to a ground storage tank or vehicle tank, such as for a space vehicle, and the liquid to be densified may be recirculated from the tank to the heat exchange tank.
The system may include first and second flow control valves for metering the introduction of the first and second components of the heat exchange bath. Furthermore, the system may include at least one sensor, such as a fluid level or temperature sensor, within the heat exchange tank to supply input to a controller that operates the flow control valves. A mixer may be provided within the heat exchange tank for circulating and mixing heat exchange bath therein. Preferably, the volume of the first component of the heat exchange bath substantially exceeds the volume of the second component, and the second component is introduced through a plurality of injectors into the second component.
A further aspect of the present invention involves a method for densifying liquid including filling a tank with a densified liquid at a reduce temperature, circulating the liquid from the tank to a plurality of heat exchange tubes within a heat exchanger, and providing a two-component heat exchange bath in the heat exchanger for cooling the liquid to be densified within the exchange tubes. The method also preferably includes the step of providing a first component of the heat exchange bath having a first boiling temperature, and a second component of the exchange bath having a second boiling temperature lower than the first boiling temperature. Desirably, both the first and second boiling temperatures are lower than the boiling temperature of the liquid propellant.
In one specific embodiment, the liquid to be densified is oxygen, the first component of the heat exchange bath is an inert liquid, and second component has a boiling temperature that is substantially lower than the boiling temperature of the first component. The first component may be nitrogen, and second component may be hydrogen. The method further may include injecting the hydrogen along separate flow paths into the heat exchanger to prevent localized freezing of the nitrogen.
The present invention, together with additional features and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying illustrative drawings.